Electrical resistance



oct. 23, 192s. 1,688,381

H. M. DOWSETT El' AL ELECTRICAL RESISTANCE Filed June 16, 1925 l/f/g. 2

gvwwboz HARRY MLVIU WSE HERBERT BHUMMN TILL @EMM femm Patented Oct. 23,1.9.28.

UNITED ,STATES HARRY MELVILLE DOWSETT,

CHELMSFORD, ENGLAND, OF NEW YORK, N. Y.

PA'ri-:Nr ori-"ics,

OF COLcHEsTER, AND HERBERT BENJAMIN TILLEY, OF ASSIGNORS TO THE RADIOCORPORATION OF AMERICA,

ELECTRICAL RESISTANCE.

Application led .Tune 16, `1925, Serial No. 37,461, and in Great BritainJune 17, 1924.

There is at present in general use a compact forni of protectedresistance unit which consists of a tube of unglazed porcelain Woundwith bare resistance wire which is sometimes oxidized and coated withvitreous glaze.

In the process of glazing, the tube is heated to a very'high temperatureso that no insulating or protective covering such as silk or enamel canbe used on the Wire, and the metal 0 employed for the wire must be suchthat it can be heated .to this temperature without melting or chan gingits character. The num,- ber of resistance alloys which can belemployedfor this purpose, is, therefore, strictly limited. particularly whenWires of very small gauge are used` and it is found that alloys whichhave a very low or negligible temperature co-efficient cannot beemployed. The porcelain tube and the finishing, glaze are bot-h porous,so that the effectiveresistance, particularly of the high resistanceunits. varies' with the moisture absorbed. The glaze as it is heatedloses its insulating properties. and as in the process of manufacr turcthe wire sometimes alters its position on the tube, and the oxidecoating if employed is not a perfect insulator and is often patchy incharacter. the winding suffers from short 'circuited turns.

The object of this invention is to obviate these disadvantages, andaccording to this invention a resistance is constructed as follows (a)The tube is made of a refractory material which is impervious tomoisture, such as glazed porcelain or glazed earthenware, or silica, ora suitable resistant glass.

(b) The wire is formed of any one of the well known resistance alloys,preferably one with a negligible temperature co-oflicient.

The Wire is enamel covered and then coated with a layer of silk, cottonorother absorbent material.

(c) The winding is impregnated With/a suitable enamel and stoved forseveral hours at 150 C. r (d) The unit is then given a second coating ofthe same enamel and stoved for a longer period at a higher temperature.

e) The second'coating may be the finishf ing coat. or a third coatingmaybe applied forthispurpose, the tube being stoved finally at atemperature higher than the maximum Working temperature, but such thatt-lie enamel does not smoke or crack during the stoving process, 250 C.being suitable.

.A better understanding of the invention v. will be had from thespecification when read in connection with the accompanyingdrawings inwhich:

Figure 1 shows in elevation a resistance unit made in accordance withthis invention, Figure 2 shows an end elevation of the unit shown inFigure 1,

Figure Bshows at a larger scale a longitudinal section taken on thecenter line of Figure 1, While Figure 4 shows in detail the successivesteps taken in constructing a unit according to our invention.

Referring to these figures, l is a tube of refractory material which isimpervious to moisture, and 2 an enamelled resistance Wire having anegligible temperature co-eflicient and covered With absorbent material10 such as silk or cotton.

In order to form the end connections, only one'of which is shown, ashort length of p11- able braided copper 3 is laid on the surface of thetube l as shown in Figs. 2 and 3,'and lashed thereto by bare copper Wire4 for a distance of about ll, the said Wire 4 having entwined with it alength of about 2 feet of resistance Wire 2 which has been bared. Thebraided copper 3 is then turned back and soldered as shown at 5, thesolder having a specially high melting point. The unit is then enamelledand stoved, as previously described, both layers of the enamel beingindicated in Fig. 3 by the numeral 6. The enamel 6 is composed of thelayer of enamel 11 which is stoved at 150 Vand a second layer of enamell2 which is then applied and stoved at a higher temperature.

7 is a clip, fitted round each end of the unit, and having a clampingscrew 8 to which the braided copper 3 is secured as indicated in Figs. land 2.

Such an end connection makes a good mechanic-al and electrical joint,and the flexible enamel coating 6 prevents any tendency of the solderto-flow when the unit is under load.

It is found that the resistance of a unitmade in this manner variesbutvery slightly, if at all', with `increase `of temperature; it can be runcontinuously at a normal temperature of 200O C. to 220 C.; there is nofailure of insulation between turns; the unit is perfectlynon-hygroscopic and is therefore suitable for ing an enamelled Wirecovered Withabsorbent material about a body of refractory material,

impregnating the resistance unit so formed with enamel and stoving it,and subsequentl y applying .a second coating of enamelto the resistanceunit and stoving it at a temperature higher than the maximumworkingtempera* ture.

2. The process of manufacturing an electrical resistance unit whichconsists'in Winding an enamelled wire covered with absorbent materialabout a body of refractory material, impregnating the resistance unit soformed with enamel and stoving it at a temperature below the normalWorking temperature, and subsequently applying a second coating ofenamel to the resistance unit and stoving it at a temperature higherthan the maximum working temperature. l.

. 3. The process of manufacturing an electrical resistance unit whichconsists in Winding an enamelled Wire covered With absorbent materialabout a body of refractory material, impregnating the resistance unit soformed 3 with enamel and stoving itata temperature between 130 C. and170 C. and subsequently applying a second coating of enamel to theresistance unit and stoving it at a temperature between 230 and 270 C.

4. The process of manufacturing an electrical resistance unit Whichconsists in winding an enamelled Wire covered with absorbent materialabout a body of refractory material, impregnating the resistance unit soformed With insulating material and stoving it and subsequently applyinga second layer of insulating material to the resistance unit and stovingit at a temperature higher ythan the maximum Working temperature of theunit.

5. The process of manufacturing an electrical resistance unit Whichconsistsin Winding an enamelled wire covered Wit-h absorbent materialabout a body of refractory material. impregnating the resistance soformed with a plurality of layers of insulating material stoving one ofsaid layers at a temperature below the normal Working temperature andanother of said layers at a higher temperature than the maximum Workingtemperature.

In testimony that We claim the foregoing as our invention we have signed'our names this fourth day of J une, 1925.

HARRY MELVILLE DOWSETT.

HERBERT BENJAMIN TILLEY.

